1. Field of the Invention
The present invention relates to a luminous element such as a light emitting diode, a semiconductor laser diode or the like and, particularly to a process for producing a semiconductor a luminous element or opto-electronic device comprising a crystal layer including a single crystal of III-group nitride semiconductor (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N to which at least one of II-group elements such as magnesium (Mg), zinc (Zn) or the like is added, which is attracted as a wide gap semiconductor expected to be a material for a device for being capable of emitting a blue light.
2. Description of the Related Art
The III-group nitride crystal made of aluminum (Al), gallium (Ga), indium (In) and nitrogen (N) i.e., [(Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1)] to which at least of II-group elements such as Mg, Zn or the like is added is produced by a chemical vapor deposition. This so-called II-group added III-group nitride as it is immediately after growth has a high resistance. In addition, the II-group added III-group nitride does not have a proper emitting property of blue light or ultraviolet (UV) radiation. Therefore, if a blue light emitting diode is produced, there is known as its emitting property being no good.
Recently, a reforming method have been reported that a high resistance (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) crystal to which Mg or Zn is added is reformed to a low resistance p-type crystal by means of a specific treatment. H. Amano et al. discloses that a low resistance p-type crystal is achieved by performing a low energy electron beam irradiation treatment to such a crystal (H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, Jpn. J. Appl. phys. Vol. 28, 1989, pp-L2112-L2114). Further, S. Nakamura et al. also discloses that a low resistance p-type crystal is achieved by performing a heat treatment under a pressurized or atmospheric pressure in an atmosphere of nitrogen to such a crystal (S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. Vol. 31, 1992, pp-L139-L142).
The low energy electron beam irradiation treatment causes an increase of a blue light emitting intensity of the element and is an excellent reforming-treatment method for achieving a low resistance crystal. However, it causes an unwanted luminescent center increase. As a result, if for instance a blue light emitting diode is produced, the diode emits a yellow light together with a blue light. This mixture emitting is problematic in color purity. This is a reason that the electron beam irradiation activates not only an inactive acceptor impurity as it is grown improving the p-type conductivity but also an yellow luminescent center. A necessary luminescent center and the acceptor impurity are not selectively activated by using the electron beam irradiation in a process for producing a light emitting diode.
On the other hand, a low resistance crystal layer is achieved by a heat treatment in an atmosphere of nitrogen. However, this heat treatment activate the acceptor impurity originating the p-type conductivity, and the increase of intensity of blue light emission from the resultant element is inferior to that reformed by the electron beam irradiation. Therefore, this heat treatment is problematic. In addition, a common heat treatment generally heats a growth layer. Since a time period for heating the crystal is long, the rising time and descending time in temperature can not be ignored in the heating treatment. The mutual diffusion of major elements and acceptor impurities between the grown layers is likely to occur in the element. This mutual diffusion is particularly problematic to a bluish green, blue or UV light emitting diode or a semiconductor laser diode each necessitating a steep interface between the adjacent grown layers.